In general, a function of preventing deterioration in the quality of contents is required of packaging materials. Particularly, in a field of food packaging materials and the like, in which contents are susceptible to deterioration by oxidation, the packaging materials are required to have excellent oxygen gas barrier property.
At present, for example, polyvinyl alcohol (PVA) films, films formed of a partially saponified product (EVOH) of an ethylene-vinyl acetate copolymer, polyvinylidene chloride (PVDC) films and the like are commonly used as films excellent in oxygen gas barrier property in the field of the packaging materials. Of these, the PVDC films have a feature that their gas barrier property scarcely depends on humidity unlike the PVA films and EVOH films. However, when they are incinerated, chlorine gas is generated, and so they involve an environmental problem. On the other hand, the PVA films are best in oxygen gas barrier property among general synthetic resin films in a dry state. However, they involve a drawback that their oxygen gas barrier property is impaired to a great extent due to moisture absorption under high-humidity conditions, and moreover they are soluble in boiling water.
Poly(meth)acrylic acid or partially neutralized products thereof are water-soluble polymers and may be formed into films from their solutions by a casting process. The films formed of poly(meth)acrylic acid are excellent in oxygen gas barrier property under dry conditions. However, these films show strong hydrophilic nature, and are hence markedly impaired in oxygen gas barrier property under high-humidity conditions, and moreover easy to dissolve in water.
In U.S. Pat. No. 2,169,250, it is described to polymerize a methacrylic acid monomer in an aqueous solution of PVA, cast the resulting reaction mixture of polymethacrylic acid and PVA on a glass plate, evaporate the water, and then heat the dry film for 5 minutes at 140.degree. C., thereby reacting PVA with polymethacrylic acid to obtain a water-insoluble film (Example I). According to such heat-treating conditions, the resulting film can be made insoluble in water. However, any film exhibiting excellent oxygen gas barrier property under high-humidity conditions cannot be obtained by such heat-treating conditions.
On the other hand, starch is a hydrophilic polymer like the poly(meth)acrylic acid. Those having excellent water solubility can easily be formed into films from aqueous solutions thereof by a casting process. Films formed of starch are excellent in oil resistance and oxygen gas barrier property, but have drawbacks that they show strong hydrophilic nature, and are hence markedly impaired in oxygen gas barrier property under high-humidity conditions, and moreover are poor in mechanical strength and water resistance.
Some proposals have recently been made for producing films or sheets from mixtures of starch and various thermoplastic resins (for example, Japanese Patent Application Laid-Open Nos. 90339/1992, 100913/1992, 114044/1992, 114043/1992, 132748/1992, 93092/1993 and 92507/1993). However, these films are still insufficient in water resistance or oxygen gas barrier property under high-humidity conditions.
The present inventors repeatedly carried out an investigation with a view toward producing a film excellent in water resistance and oxygen gas barrier property under high-humidity conditions. As a result, it was found that when a film is formed from a mixture of poly(meth)acrylic acid and/or a partially neutralized product thereof and a saccharide such as starch, and the resulting film is subjected to a heat treatment under specific conditions, a film exhibiting remarkably improved oxygen gas barrier property even under high-humidity conditions, to say nothing of dry conditions, compared with films separately formed from the individual components can be obtained (U.S. patent application Ser. No. 08/324,541, now U.S. Pat. No. 5,498,662). However, the simple film thus obtained is insufficient in sealing property, mechanical strength, moisture resistance, etc., which are required of packaging materials. There has thus been a demand for a further improvement.